Apraxia of speech is a neurological disorder that affects the brain pathways involved in producing speech. Even though children know what they want to talk about, their brains have a tough time planning the movements needed to make the words. It's like knowing exactly what you want to draw, but finding that your hand can't quite move the way you want it to with the crayons.
For kids with Apraxia, speaking clearly is something that doesn't come easy and involves a lot of repetition, like how you might practice tying your shoes or riding a bike.